1. Field of the Invention
The present invention relates to a damper valve for a hydraulic power steering device and, in particular, to a damper valve for a hydraulic power steering device where damping performance is improved so that operator steering feeling is enhanced.
2. Description of the Related Art
A conventional damper valve for a hydraulic power steering device used for a vehicle has been proposed, such one as shown in FIG. 7 (refer to Japanese Patent Application No. 10-159665).
In this proposal, a damper valve 030 is provided integrally with a casing 02c of a gear box accommodating an oil passage changing-over valve (not shown). It indudes a partition plate member 050 for partitioning the interior of a valve casing 031 into two chambers and a shaft member 051 extending through a central portion of the partition plate member 050.
A first valve mechanism 052 for allowing only flow of hydraulic oil returned back to the oil passage changing-over valve from either one of left and right oil chambers of a power cylinder is provided in the partition plate member 050, while a second valve mechanism 053 for allowing only flow of hydraulic oil advancing to either one of the left and right chambers of the power cylinder from the oil passage changing-over valve is provided in the shaft member 051.
The first valve mechnism 052 comprises a plurality of communication holes 054 formed in the partition plate member 050 and a valve spring 055 for opening/dosing the plurality of communication holes 054, and the second valve mechanism 053 comprises a ball type check valve provided with a ball 056 and a one-way clip 057.
With a conventional damper valve configured in the above manner, when hydraulic oil flowing from the oil passage changing-over valve towards either one of the left and right oil chambers of the power cylinder opens the ball type check valve to flow through the second valve mechanism 053, the flow is subjected to resistance from the ball type check valve, so that kinetic energy of the hydraulic oil is lost to some extent.
Also, when hydraulic oil flowing from either one of the left and right chambers of the power cylinder towards the oil passage changing-over valve opens the valve spring 055 to flow through the first valve mechanism 052, the flow is subjected to resistance from the valve spring 055, so that kinetic energy of the hydraulic oil is lost to some extent.
When vibrations are transmitted through damping actions of both the first valve mechanism 052 and the second valve mechanism 053 from a road wheel side (not shown) of a vehicle to the power cylinder via a tie-rod and a rack shaft during running of the vehicle, the vibrations are damped to be prevented from being transmitted to the steering wheel via the oil passage changing-over valve. Also, since good responsive feeling is obtained in a neutral position of the steering wheel, appropriate operator steering feeling is maintained.
However, in the conventional damper valve for a hydraulic power steering device, the damping characteristic depends only on change in an oil passage opening area due to deformation of the valve spring 055 of the first valve mechanism 052.
Therefore, as understood from the damping characteristic shown in FIG. 8, there is a drawback that, as sliding speed (rack sliding speed) of the rack shaft becomes faster due to a hydraulic oil pressure difference between the left chamber and the right chamber of the power cylinder increasing during a low speed running of the vehicle, forces acting from a road surface so as to resist the sliding of the rack shaft, namely reverse sliding load, is increased so that a high friction feeling and/or a balky feeling in steering exists during a low speed running of the vehicle in some cases.